Plastic containers have been increasingly used in packaging applications, such as “food packaging”, and in particular for cook-in convenience food. As an example, ready-prepared ovenable meals can be provided which can be warmed in a conventional oven. Today, busy customers are demanding higher quality products, easier-to-use packaging and faster reheating options to achieve more flexibility in the kitchen. The ability to cook food in the conventional oven is a practical necessity.
Thermoforming is a known packaging process in which a container (e.g. a tray) is formed by a plastic film in a mold by application of vacuum, air pressure or a plug under increased temperature. Then, food is placed in the container and air is drawn from the package prior to sealing it close. Such packaging has comparable storage and preservation characteristics to shrinkable packaging
Thermoforming process in general involves two packaging films:                1. The top lid film which seals under vacuum to the bottom film.        2. The bottom thermoformable film which is formed into a tray during the first step of the packing process and where-into the food is placed prior to the vacuum-seal step.        
Cook-in packaging is packaging in which a food product is packed non-cooked or precooked for a consumer. The consumer then warms or cooks the food without removing the package.
The cook-in concept is particularly desired since it avoids the need for the consumer to handle raw meat or fish, which is often disagreeable. Moreover, the handling of raw meat is a growing concern from a food safety perspective while a pre-packed cook in food package reduces the risk of contamination.
In case the cook-in is done in a conventional oven, the films or trays used must combine the following features:                1. Enough thermal resistance not to disintegrate in the oven for a cooking process of 200° C. lasting 1 hour.        2. The sealing of the top lid film to the bottom thermoformable film must be strong enough to avoid any leakers or unsealed areas which could be dangerous for the food integrity.        3. The oxygen barrier of both top and bottom (or tray) materials are high enough to protect the film from oxygen ingress which is detrimental for the shelf life of the product.        
In general, common films used in thermoforming are multilayers incorporating different layers of polyamides, polyolefins (e.g. polyethylene or polypropylene) and optionally higher barrier materials such as EVOH. While these combinations of materials are very well performing in conventional applications, they do not have sufficient heat resistance to withstand conventional oven temperatures like 200° C. They often disintegrate during this kind of cooking.
In the prior art several approaches have been used to increase the thermal stability of the thermoforming films. U.S. Pat. No. 7,504,158 describes a film comprising mostly polyamide. While this film is heat resistant enough, its sealing behavior in the thermoforming machine is not sufficient and unstable. Same deficiencies stand with the films described in US 2012/0213896.
In the market, there are films comprising polyamide 66 that are used in similar applications. Polyamide 66 has a very high melting point at about 255° C. and it is not easy to extrude in conventional extrusion lines. It is also very stiff. The present application is able to offer improved performance with polyamides having a melting point up to 240° C.